Answer
An alluvial fan forms when a fast-flowing stream or river exits a narrow valley and enters a broader, flatter area, such as a plain or a basin. As the river leaves the narrow valley, it loses energy and velocity, causing it to deposit the sediment it carried from the upper reaches. The sediment is spread out across the broader area, forming a fan-shaped deposit. Alluvial fans often have a characteristic fan-like appearance, with coarse sediment near the apex of the fan and finer sediment farther away from the source. The formation of alluvial fans is common in arid and mountainous regions where fast-flowing streams carry a significant amount of sediment and encounter flatter terrain.
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